Messed up screen - need helpl

[tep392]

I need a little help here. Al was testing my 5200 conversion of Zaxxon and the intro screen is a little messed up on his 5200. I don't see the same problem on my 5200 so it could be some difference in the machines. His is an A/V modded 4-port while mine is stock 2-port. His other games work fine though, so it's possible I'm doing something wrong in the code. Maybe I'm initializing something incorrectly? Any suggestions?

 

Al's screen.

 

It should look like this

 

[Albert]

I'm going to test this same binary on two other 5200 systems later this afternoon. I know the game works fine on one of them, as i had the game running at the Classic Game Fest back in July without any issues. I'll report back here once I've done that.

 

The game appears to run fine, as far as I can tell. It's just the title screen that is wrong for some reason.

 

..Al

[Kr0tki]

Al's screen looks as if displayed with the GTIA mode 11 enabled. Are you initializing PRIOR correctly?

[tep392]

I'm writing #$11 to PRIOR. This value gets copied from the shadow register to PRIOR during the vertical blank. I checked it running in the emulator and it seems to be writing to the register correctly.

[tep392]

I think that was it. Checking a bit more in the emulator, it looks like I'm not setting the value in the shadow register until after the title screen finishes. I didn't catch it the first time it ran. Each time after that, the shadow register is retained during the "cold start" in the emulator. I'm not sure why it seems to work fine on my 2-port though.

 

Thanks kr0tki!

[Kr0tki]

Late 2-port units have a different BIOS revision, maybe this is somehow related. Try testing both BIOS-es under emulation.

[tep392]

I tried that both BIOS's and it didn't make a difference. I think the root cause is I'm not clearing the shadow ram when the game boots so it's writing whatever happens to be in GPRIOR until I update it after the title screen.